This invention relates to a method of damping surges in liquid systems.
It is known in a pumped-liquid system to utilise a surge vessel in which there is provided a trapped volume of gas, usually air; so that in the event of a surge, the trapped air volume in the vessel damps the surge. Surge conditions may occur for various reasons, typically but not limited to normal liquid pump start/stop sequences, pump failure, power supply failure and general control and valve failure in the liquid system, or even as a result of a major liquid flow valve being opened or closed, or rapidly changing demand.
It will be appreciated that in a pumped-liquid system such as a mains or foul water system, the pumping regime is varied in accordance with varying demand i.e. the pumped volume is varied. Accordingly, conventionally to accommodate different pumping regimes, air is introduced into or released from the surge vessel thus to maintain generally constant the liquid level within the vessel. This is achieved by means of air inlet and outlet valves controlled by a control means which is sensitive to the liquid level within the vessel, and an air compressor.
However, in use, particularly where the trapped air is at high pressure, some of the air tends to dissolve in the liquid in the surge vessel, and thus periodic xe2x80x9ctopping upxe2x80x9d of the air within the surge vessel is required, and each time the pumping regime changes, air has to be either introduced into the surge vessel by operating the air compressor and inlet valve, or released from the surge vessel by operating the air release valve thus to maintain generally constant the liquid level within.
Frequent operation of the air inlet valve and compressor, and the outlet valve as the liquid level within the surge vessel changes, is undesirable, and so as not to introduce into or release from the surge vessel too much air, adjustments have to made slowly, which can be problematic where pumping regimes are frequently changed. Moreover, upon releasing excess air from the surge vessel, noise can be generated which is environmentally undesirable.
According to a first aspect of the invention we provide a method of damping surges in a liquid system, the method including providing in the system, a surge vessel in which there is trapped air or other gas, the surge vessel being positioned so that in the event of a surge in the system, the volume of trapped air or other gas within the surge vessel changes to damp the surge, the method being characterised in that the mass of the trapped air or other gas in the surge vessel is maintained generally constant, irrespective of the liquid level within the surge vessel
It will be appreciated that by using the method of the invention, there is generally no need to release air or other gas from the surge vessel to maintain the damping effect of the surge vessel e.g. in a pumped-liquid system, for different pumping regimes, and moreover, there is no need to xe2x80x9ctop-upxe2x80x9d the air or other gas within the surge vessel to accommodate different pumping regimes. Thus the air or other gas compressor usage may be minimised, and moreover there is no need to operate an air or other gas release valve to release air or other gas from the surge vessel to accommodate different pumping regimes. Thus noise generated by the method of the invention is generally reduced compared with a conventional system.
Utilising the invention, regardless of the pumping regime, the mass of the air or other gas trapped within the surge vessel is maintained constant. Otherwise, to damp surges in the pumped-liquid system, the surge vessel is operated substantially conventionally, with the trapped air or other gas acting on the liquid in the surge vessel to damp the surges in the liquid.
It will be appreciated from Boyle""s gas law that the product of pressure and volume is proportional to the mass of the trapped air or other gas. Thus by sensing the pressure and volume of the trapped air or other gas, a control means may be utilised to respond to changing trapped air or other gas mass. In practice, the mass of the trapped air or other gas is most likely to decrease as air or other gas is dissolved in the liquid in the surge vessel, so that the volume of the trapped air or other gas will decrease.
Thus the method may include responding to a decrease in the volume of the trapped air or other gas, to introduce more air or other gas into the surge vessel to restore the mass of the trapped air or other gas to a set value.
The method may include sensing the pressure of the trapped air or other gas in the surge vessel and determining a measure of the volume of the trapped air or other gas, and providing respective inputs to a control apparatus which responds to the inputs to maintain the mass of air or other gas trapped in the surge vessel generally constant.
In one example, a measure of the volume of the trapped air or other gas may be determined by monitoring the liquid level, in the surge vessel. Thus by knowing the volume of the surge vessel which will be constant or substantially constant, a measure of the volume of the trapped air or other gas may be determined.
For example, the liquid level in the surge vessel may be monitored by determining a differential pressure between the pressure of air or other gas trapped in the surge vessel and the pressure exerted by the mass of liquid in the surge vessel. From this differential pressure, again by knowing dimensions of the surge vessel, the volume of liquid and hence of the trapped air or other gas may be determined.
The control apparatus may be arranged to respond to inputs which indicate a decrease in the mass of the trapped air or other gas, by operating an air or other gas compressor and introducing more air or other gas into the surge vessel until the mass of the trapped air or other gas again attains a set value.
According to a second aspect of the invention we provide a control apparatus for use in damping surges in a liquid system, the system including a surge vessel in which there is trapped air or other gas, the surge vessel being positioned so that in the event of a surge in the system, the volume of trapped air or other gas within the surge vessel changes to damp the surge wherein the control apparatus includes a processor which responds to inputs from a sensor apparatus to determine a measure of the mass of the trapped air or other gas in the surge vessel, and a pump for introducing air or other gas into the surge vessel to maintain the mass of trapped air or other gas generally constant.
In one embodiment, the control apparatus includes a first sensor such as a pressure transducer, to sense the pressure of the trapped air or other gas in the surge vessel and to provide a first electrical input to the processor, and a second sensor, again such as a pressure transducer, to sense the pressure exerted by the liquid in the surge vessel, and to provide a second electrical input to the processing means.